The present invention relates to a fuel supply system for a turbocharged internal-combustion engine, for example an indirect-injection turbocharged diesel engine.
Turbocharged engines tend to suffer an acceleration lag as the turbocharger speeds up to a value where it can develop the boost demanded. This problem is particularly acute with diesel engines, where low exhaust temperatures at an initial low load level and the action of the boost control unit combine to limit the rate at which the increased energy required to produce turbocharger acceleration can be made available in the exhaust gases. The temperature of the exhaust gases and their release pressure can be increased by retarding the fuel-injection timing, but this has the effect of worsening fuel consumption and, with most diesel engines, causing excessive exhaust smoke. The inconvenience of this transient turbocharger lag problem is most troublesome in diesel-powered passenger cars, where continuous changes of load and speed are required in traffic.
Indirect-injection combustion systems are generally used for diesel-engined cars. These systems have the feature that as much as 60% of the combustion chamber space at firing TDC is contained in a cell which is separate from the volume in or immediately above the piston crown. Such separate cells are usually spherical in shape and air is forced into them by the piston, via a tangential throat, during the compression stroke. Combustion is initiated and sustained by the timed injection of fuel into the combustion cell.
In one known distributor-type fuel-injection pump which is used on small, high-speed diesel engines, the driving shaft (operating at half engine crankshaft speed, as is usual for a 4-stroke engine) drives a vane-type fuel pressurizing pump and, via splines, also rotates a single high-pressure distribution plunger. The plunger has a timed reciprocating axial movement superimposed on its rotary motion by a system of rotating cams and relatively fixed rollers.
The vane pump compresses the fuel to an intermediate pressure and is connected to an inlet port in the distribution plunger casing. The motion of the plunger causes fuel to be entrapped and sequentially delivered to each cylinder in turn at the appropriate time. The injection period has a fixed start point but a variable ending, depending upon a control setting which may be determined by a fuel control lever, or by a governor if under automatic control.
Since the volume of fuel delivered by the vane pump increases with speed, the intermediate pressure generated also rises (within the parameters set by the relief valve). Use is made of this rising intermediate pressure with speed to adjust the angular position of the fixed rollers in order to advance the start of fuel injection as the engine speed rises. This characteristic frequently suits combustion requirements.
In another known distributor-type fuel injector pump, a central shaft or rotor of significant diameter is driven at half engine speed. Two opposing pump plungers are located in radial bores in the rotor. These plungers are forced outward by the centrifugal force generated by rotation of the rotor, and they are moved radially inward by a fixed cam ring located around the rotor, the cams being so shaped as to provide the desired fuel injection rate.
A vane pump also driven by the rotor supplies fuel at an intermediate pressure to a passage having a spring-loaded relief valve at one end and a metering valve at the other. The metering valve has the controlled pressure from the vane fuel pump acting on one end of its plunger while the other end thereof bears against a spring whose opposite end is positioned by a control lever operated by either a governor or the accelerator pedal. Thus, the position of the metering valve is controlled by the load-dependent spring at one end and the intermediate level of pressure delivered by the pump, which is fixed by the pressure relief valve. The metering valve delivers fuel to an inlet in the rotor casing and, via a series of passages in the rotor, fuel is conveyed sequentially to the space between the plungers. The plungers sequentially pressurize and deliver the fuel, via an outlet in the rotor and a series of outlets in the rotor casing, to the different cylinder fuel-injector nozzles.
The metering valve determines, for a fixed transfer pressure, the rate at which fuel enters the high-pressure pumping space between the two radial plungers in the time available between successive injections. Angular movement of the rotor cuts off the single inlet just before the cam forms start to move the plungers radially inward. Depending on the degree to which the space between the plungers has filled with fuel, a point is reached as the plungers lift when any empty space has been reduced to zero and the fuel becomes "solid," whereupon its pressure rises with further inward movement of the plungers. At this point, the single outlet at the far end of the rotor overruns one of the fuel delivery outlets in the casing so that the now-pressurized fuel is then delivered to the appropriate engine cylinder.
It is clear that at small fuel deliveries the plungers will have lifted some way towards their innermost position before the entrapped fuel becomes "solid" and its pressure rises. As a result, the start of injection is later at light engine loads than at full load, i.e., the start of injection varies with load. Delivery ends when the plungers reach the tops of the cams, i.e., the timing of the end of injection is constant with respect to the cam lift.
In practice, the "fixed" cam ring is angularly located by a lever whose position is determined in dependence upon the reaction to the driving torque imposed on the cam ring. Thus, the greater the quantity of fuel injection, the greater the torque reaction and the greater the angular movement of the cam ring against the resisting torque imposed by a spring. Dependent on the spring load and rate, the actual start of injection can be varied as required with engine load to give, for example, a constant start of injection timing with respect to engine TDC.
In addition, since the pressure delivered by the vane pump is dependent on the fuel quantity pumped by its rotor, varying directly with engine speed and with the characteristics of the relief valve spring rate and opening area, the fuel transfer pressure rises with engine speed. Consequently, the position of the cam ring is speed-sensitive and as a result injection can be advanced with speed, as desired.
So far the operation of these pumps has been briefly described for a normally-aspirated engine, for which a fixed stop is used to limit the maximim quantity of fuel which can be injected, to avoid excessive exhaust smoke. In the case of a boosted engine, the maximum amount of fuel which can be burned increases with the boost pressure, so it is usual to have a device operated by the boost pressure which automatically alters the fuel pump maximum delivery stop in accordance with the prevailing manifold boost pressure.